In order to convert interlaced-scanning video signals into progressive-scanning video signals or convert progressive-scanning video signals into enlarged or reduced video signals, an interpolation circuit that carries out scanning lines interpolation is used. In the interpolation circuit, the value of a pixel to be produced by the interpolation (hereinafter referred to as “interpolation pixel”) is calculated based on the values of pixels around the interpolation pixel. The angle of a picture having a diagonal edge or a thin diagonal line for example is detected based on the luminance distribution of surrounding pixels and the value of the interpolation pixel is calculated using pixels in the direction with a higher degree of correlation.
Japanese Patent Laid-Open No. 9-37214 discloses a progressive scanning lines interpolation apparatus that can carry out scanning lines interpolation in a picture having a diagonal edge.
The progressive scanning lines interpolation apparatus selects candidate sets of pixels for calculating the differential absolute value between pixel values from sets of original pixels located point-symmetrically around an interpolation pixel in order to convert an interlaced-scanning video signal into a progressive-scanning video signal. The differential absolute values between the values of the selected sets of pixels are each calculated. Then, based on the edge information of these sets of the original pixels, the differential absolute values are corrected, a set of original pixels whose corrected differential absolute value is minimum is detected, and the interpolation pixel is produced based on the detected set of original pixels.
In this way, a picture having a diagonal edge can be subjected to scanning lines interpolation.
The conventional progressive scanning lines interpolation apparatus however cannot determine whether the edge of the picture is a straight line shape or a curved shape such as an arc. Therefore, a picture having a curved edge such as an arc shape cannot be formed into a smooth picture by interpolation.